An apparatus and method for separating hydrocarbons and other organic materials from water, such as acidflow back water, completion fluid water, produced water and rain water collected from off-shore oil drilling and production platforms, are disclosed. More specifically, an apparatus is disclosed for pre-treating these production waters generated on off-shore oil drilling and production platforms by injecting the production water in an enclosed vessel and generating a cyclonic flow within the vessel. Recycled water from the vessel and a sparge gas are also tangentially injected into the vessel to further encourage the cyclonic flow. The gas and hydrocarbon-rich water migrate towards the top surface of the liquid in the vessel and towards the axial center of the vessel where the hydrocarbon-rich water is purged. Hydrocarbon-lean water is then purged from a bottom section of the vessel and discharged or processed further.
During crude oil production, a significant amount of water is co-produced with the oil. This xe2x80x9cproduced waterxe2x80x9d is contaminated with residual or hydrocarbons and therefore is a substantial waste stream. For example, it is estimated that 380 million tons of produced water was generated in the North Sea during the 2001 production year. While new operational fields produce relatively minor amounts of water, e.g., 10-20% percent of the total production (i.e., water and oil), as an oil field ages, the produced water volume increases to 80-90% of the total production. These huge volumes of produced water must be treated before they are returned to the sea because they contain significant amounts of hydrocarbon contaminants.
Currently, off-shore production facilities treat the hydrocarbon-contaminated production water by adding oil coalescing and water clarifying chemical agents to assist in a mechanical separation of the hydrocarbons from the water. However, this technology results in the discharge of production water to the ocean which still contains hydrocarbons in the range of 20-40 ppm and additional trace impurities such as benzene related compounds, phenols, alkyl phenols and polyaromatic hydrocarbons in concentrations ranging from 100 to 10,000 ppb. While a 20-40 ppm hydrocarbon content meets current regulations, it is been found that the discharge of production water with the above-noted trace impurities can adversely affect marine life.
Often, large amounts of natural gas are produced with crude oil. To separate the gas from the oil and the oil from the water, three phase separators have been developed. In a three phase separator, the gas is first separated from the oil and water and the oil layer is physically separated from the water and sent to a dehydrator to remove residual water. The water phase, which includes a small fraction of residual oil, enters a water skimmer to skim the free oil off of the top of the water layer. After skimming, the water layer, which still contains a substantial amount of hydrocarbons, enters a horizontal induced gas floatation separator. These horizontal induced gas floatation (HIGF) separators can produce a water phase with a hydrocarbon content of 20-40 ppm.
HIGF separators work by bubbling a gas through the production water which results in hydrocarbon droplets floating to the surface. Typically, the gas used is natural gas produced at the well. Nitrogen or other inert gases may be used as well. Each HIGF separator includes a number of cells, each with its own gas diffuser to maximize the natural gas bubble/hydrocarbon droplet contact. While the HIGF separators are commonly used, they suffer from numerous drawbacks.
At the outset, HIGF separators are extremely large. Their length can reach 60 feet which represents a tremendous amount of deck space, which is at a premium on an off-shore oil platform. Many older platforms, where space is limited, cannot be outfitted with such separators. It will be noted that all currently available xe2x80x9chorizontalxe2x80x9d induced gas floatation separators have a length or width substantially greater than their height.
Further, HIGF separators to date have not been able to reduce hydrocarbon concentration below an approximately 20 ppm threshold level. While this level meets current regulatory standards, it falls short of the proposed standards for the North Sea which may take effect as early as 2005.
HIGF separators are also susceptible to wave motion experienced by modern platforms. Specifically, modern deep water platforms are not permanently anchored to the sea floor but, instead, are tethered and move with currents. Thus, these floating platforms will sway and roll with wave motion. This rolling causes the water inside the HIGF separators to form waves, which makes skimming hydrocarbons from the water surface difficult and often ineffective. Further, intense wave motion will cause some units to shut down thereby creating disruptions in platform production because of a lack of storage capacity for untreated production water.
Also, because of their large size, in the event the production water output exceeds expectations, operators are unable to expand capacity by adding additional HIGF separators because of a lack of available floor space. As a result, either the operator must reduce production or discharge produced water with a hydrocarbon content greater than the regulatory limit.
Further, the flow rate of production water from a well can vary greatly and HIGF separators operate more efficiently in a steady state condition and the efficiencies of these systems is compromised with varying input flows. Still further, HIGF separators are limited in their ability to treat water with higher hydrocarbon concentrations, i.e. greater than 300 ppm. Concentrations exceeding 300 ppm generally exceed the separators ability to achieve acceptable hydrocarbon removal. HIGF separators are also designed to remove dispersed oil or hydrocarbon droplets. Their ability to remove partially soluble components such as alkyl phenols and polyaromatic hydrocarbons is extremely limited as these components are relatively soluble in water and do not respond to gas/bubble contact. As noted above, these compounds are extremely harmful to marine life.
One substitute for HIGF separators has been suggested in the form of a Vertical vortex separator. One example is disclosed in WO 99/00169. The disclosed apparatus relies upon creating a vortex in a cylindrical vessel for purposes of separating oil from water. However, this apparatus is suitable only for preliminary separation of oil from production water and does not reduce the hydrocarbon content in the production water to a level acceptable for discharge or more intensive treatment such as filtering with organophillic clays.
The treatment of production water with organophilic clays is also known and is disclosed by commonly owned U.S. Pat. Nos. 5,935,444, 6,398,966 and 6,398,951, all of which are incorporated herewith. The production water is typically introduced into a contained vessel which contains a plurality of cartridges containing the organophilic clay. The production water flows through the packed cartridge beds of organophilic clay and the hydrocarbon contaminants are adsorbed onto the clay particles. The process is very efficient, resulting in extremely low hydrocarbon content of the treated production water.
However, it has been found that when the hydrocarbon content of the production water inputted into an organophilic clay containing vessel exceeds 100 ppm, the available adsorbing sites on the clay are readily used up and the cartridges must be replaced frequently, thereby increasing costs and creating time delays. The vortex creating apparatus of WO 99/00169 is not suitable as a sole pre-treatment of production water upstream of an organophilic clay filtering apparatus.
Therefore, there is a two-fold need for improved methods of treating production water, especially on off-shore oil platforms. First, an improved method and apparatus is required which avoids the disadvantages of the HIGF separators described above. Further, there is a need for an improved hydrocarbon/water separation method and apparatus which can be used as an effective pretreatment prior to additional treatment of the water with organophilic clay cartridges.
An improved apparatus for separating hydrocarbons from water is disclosed. The apparatus comprises a vessel having a height and a diameter. The height of the vessel is greater than the diameter of the vessel thereby providing it with a xe2x80x9cverticalxe2x80x9d configuration which is preferred by off-shore oil platform operators. The vessel further comprises an enclosed top and bottom with a vertical cylindrical section extending therebetween.
At least one input inlet is provided that extends through the vertical cylindrical section. The input inlet is connected to a supply of input liquid, i.e., untreated production water. The input inlet is directed xe2x80x9ctangentiallyxe2x80x9d or at an angle of less than or equal to 45xc2x0 with respect to a tangent of the vertical cylindrical section so that the input inlet can encourage or generate a cyclonic flow within the vessel.
The apparatus further includes at least one recycle fluid/sparge gas inlet that extends through the vertical cylindrical section of the vessel as well. The recycle fluid/sparge gas inlet is connected to a recycled pump which, in turn, is connected to the vessel by a recycle line. The recycle fluid/sparge gas inlet is also connected to a source of sparge gas. Similar to the input inlet, the recycle fluid/sparge gas inlet is also directed xe2x80x9ctangentiallyxe2x80x9d or at an angle of less than or equal to 45xc2x0 with respect to a tangent of the vertical cylindrical section for generating or encouraging cyclonic flow within the vessel.
The apparatus also includes a hydrocarbon-lean water outlet. The apparatus further includes an upwardly directed collection bucket disposed along an axial center of the vessel. The collection bucket is also connected to a hydrocarbon-rich outlet line. The vessel is also equipped with a sparge gas outlet line.
In a refinement, the apparatus further includes an upwardly directed recycle fluid/sparge gas inlet. This upwardly directed recycle fluid/sparge gas inlet is also connected to the recycle pump and the supply of sparge gas and is directed upwardly to generate or encourage upward flow within the vessel.
In another refinement, the at least one input inlet comprises two input inlets disposed diametrically opposite the vessel from one another. The two input inlets are also disposed at a common vertical height.
Further, three input inlets may be provided which are equidistantly spaced around the vessel, or at approximately 120xc2x0 intervals around the vessel. Again, a common vertical height is preferred.
In another refinement, the at least one recycle fluid/sparge gas inlet further comprises two recycle fluid/sparge gas inlets disposed diametrically opposite the vessel from one another and having common vertical height. Preferably, three recycle fluid/sparge gas inlets are provided, equidistantly spaced around the vessel at a common vertical height. In still a further refinement, the common vertical height of the recycle fluid/sparge gas inlets is disposed below the common vertical height of the input inlets.
In still another refinement, the input inlets are disposed approximately one foot below the surface of the circulating cyclonic flow within the vessel. The recycle fluid/sparge gas inlets are disposed below the input inlets, and preferably at or about a mid-point of the vessel.
In another refinement, a ratio of the vessel to the diameter of the vessel ranges from about 5:1 to about 1.5:1, more preferably about 2.5.
Further, as noted above, to meet increasing demanding environmental concerns, the hydrocarbon-lean water outlet may be connected to a secondary treatment vessel containing organophilic media for adsorbing any residual hydrocarbons that remain in the pre-treated production water.
A method for reducing hydrocarbon content in a stream of production water is also disclosed. The method comprises tangentially injecting the production water into a cylindrical vessel to encourage cyclonic flow within the vessel, tangentially injecting the flow of recycled water from the vessel and sparge gas into the vessel at a level below a point where the production water is injected and to further encourage cyclonic flow within the vessel, allowing the sparge gas and hydrocarbon-rich water to migrate to a top surface of the circulating liquid within the vessel, purging the hydrocarbon-rich water at a top surface of the circulating liquid and along a central axis of the vessel, and purging hydrocarbon-lean water from a lower point in the vessel below where the flow of recycled water and sparge gas are injected into the vessel.
In a refinement, the method also includes maintaining a positive gage pressure within the vessel with additional sparge gas or an inner gas. In another refinement, the sparge gas is natural gas co-produced with the production water to be treated.
An improved system for treating production waters is also disclosed. The improved system includes a vertically-oriented pre-treatment vessel where the production water is mixed with a sparge gas, such as co-produced natural gas, and a hydrocarbon-rich layer is skimmed off the top surface while a hydrocarbon-lean layer is removed through a lower portion of the pre-treatment vessel. The hydrocarbon-lean water is then transmitted to a secondary treatment vessel where it is contacted with an organophillic clay.